QUATERNARY DATING METHODS 1: RELATIVE DATING TECHNIQUES

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1 QUATERNARY DATING METHODS 1: RELATIVE DATING TECHNIQUES Objectives: In this lab we will examine relative dating methods used to date Quaternary glacial deposits. After this lab you should be able to: 1) recognize and understand the stratigraphic relationships between a multiple sequence of glacial deposits. 2) use a diverse suite of relative dating techniques (weathering rinds, soils, and lichenometry) to constrain age relationships between different aged glacial deposits. 3) recognize and understand the limitations of relative dating techniques. Key terms and concepts: Relative Dating Bracketing Age Minimum vs. Maximum Ages Radiometric (Numerical) Age Glacial Stratigraphy Weathering Rind Lichenometry Chronosequence Temporal It is important for both geologists and paleoclimatologists to be able to date Quaternary deposits in order to fully understand landscape evolution and environmental change over time. Dating glacial sediments is often a difficult task, as few datable materials typically exist in glacial deposits. In most cases only bracketing and/or relative ages are able to be assigned to a particular deposit. Bracketing ages can be determined by radiometrically dating organic matter or other datable materials (i.e., volcanic ash), which may be either directly or indirectly associated with the glacial deposits. Geologists will often use relative dating techniques such as stratigraphic relationships, comparative weathering data, and soils as a means to discriminate different aged glacial deposits. Today you will be given the opportunity to apply your knowledge of relative dating techniques to infer chronologic (age) relationships between a series of glacial deposits in the Rock Creek drainage of the Sierra Nevada California. Station 1. Glacial Stratigraphic Relationships You have been provided with a glacial geology map of the Rock Creek drainage in the Sierra Nevada, California. Seven different aged glacial moraine sequences (Tioga, Recess Peak, Sherwin, Tahoe, Tenaya, Matthes, and Hilgard) were mapped by J.H. Birman in We have a copy of Birman's map, but we have accidentally cut the map explanation into small pieces. We desperately need to have these glacial moraines relatively dated by the end of this lab session. Using your knowledge of glacial stratigraphy, provide a glacial chronology for the Rock Creek drainage. 1

2 1. List the seven glacial deposits from youngest to oldest. YOUNGEST OLDEST Station 2. Weathering Rinds A group of glacial geologists have conducted a study of surface boulders in the Rock Creek vicinity. They have found several basalt clasts in alluvial fan deposits which have been independently dated using the radiocarbon method for younger fan surfaces and fission track dating of volcanic ashes interbedded in the older alluvial fan surfaces. They have measured weathering rinds on basalt clasts excavated from shallow soil pits. Their data are presented below: Surface Age Weathering Rind Thickness 10,000 yr 0.68 mm 50,000 yr 1.8 mm 150,000 yr 2.2 mm 500,000 yr 2.75 mm 1,000, 000 yr 3.0 mm 1. Construct a weathering rind curve on the graph paper provided based on the above data (plot surface age along the x-axis and weathering rind thickness along the y-axis.) 2. Does the accuracy of this dating method increase, decrease, or remain the same as the rocks get older? 3. We have excavated similar type basalt clasts from the Tioga, Tahoe, and Sherwin moraines. Use your newly constructed weathering rind curve to assign numerical ages to the Hilgard, Tioga, Tenaya, Tahoe, and Sherwin glaciations. Write your answers below. Glaciation Weathering Rind Thickness Age Hilgard Tioga Tenaya 1.5mm Tahoe Sherwin 2

3 4. Could you use the curve you constructed to date the age of a weathered granitic clast? Why or why not would the method be applicable? Station 3. Lichenometry Lichen growth patterns have been studied in many alpine and arctic environments. Scientists have found that the diameter of lichens will increase at a particular rate over time. This growth rate is both species and climate dependent. 1. You have been provided rulers to measure lichens on surfaces of known age to establish your own lichen growth curve for the Rock Creek region. Measure the average diameter of the largest lichen at each site. Plot this curve on the graph paper we have provided. Age of surface Lichen Diameter Whiskey Glenn's Tombstone (1850 AD) Tequila Bob's Tombstone (1640 AD) Native American Stone Circle A (2500 yr ago) Native American Stone Circle B (4500 yr ago) Surface boulders of lahar flow (10,000 yr ago) Surface boulders of lahar flow (20,000 yr ago) 2. Based on the lichen growth data for this region, what is the oldest surface you will be able to realistically date using lichenometry? Why? 3. Our research collaborators measured the lichens found on some of the surface boulders of the Hilgard, Recess Peak, Matthes, Tioga, and Tenaya moraines. Using their measurements and your calibration curve, assign numerical ages to each glacial deposit. Glacial Deposit Lichen Diameter Lichen Age Tioga 19 cm _ 3

4 Matthes 2.1 cm _ Hilgard 18.5 _ Tenaya 18.8 _ Recess Peak 10.1 _ Tahoe 19.2 _ Station 4. Soils and Chronosequences Although we often use soil profiles to relatively date glacial sediments, it is important to remember that several factors influence soil formation. The main factors are: Climate (temperature, rainfall), Organisms (vegetation, worms, etc.), Relief (slope angle and direction), Parent material (bedrock, till, outwash, etc.), and Time (age of the soil). (An easy way to remember these factors is to remember the word CLORPT.) To be able to use soils as a relative dating method all factors except time must be the same. We have excavated soil pits on both the Tioga and Sherwin moraines. We have brought these samples back to the lab for your evaluation. Study the two soil profiles and discuss among your group how soil development changes with increasing age. Answer the following questions regarding your observations on soil development. 1. How does soil color change with increasing age between the B horizons of the two soil profiles? What might account for this change in B horizon color as a soil becomes older? 2. How does the depth of weathering of the younger soil profile compare with the depth of the older soil profile? 3. How does the soil texture (grain size) change with increasing age of a given soil profile? (Which has a higher clay content?) 4. How does the color of the surface horizon (A horizon) of the Tioga profile differ from its lower horizons? 4

5 What material in the surface horizon do you think accounts for this difference in color within the soil profile? 5. What are some of possible problems you might face in using soils as a relative age indicator for alpine glacial chronologies? (Hint: Think of the factors which influence soil formation.) Summary Question What are the benefits and shortcomings of using relative dating techniques such as soil development, progressive weathering data, and stratigraphic relationships in establishing glacial chronologies? Give a few examples of benefits and of shortcomings. 5